1. Field of the Invention
The present invention relates generally to a slip interface between sensor coils for fiber optic gyroscopes and mounting hubs, and specifically to an arrangement for supporting a potted sensor coil that minimizes temperature-induced Shupe effect due to fiber stressing.
2. Description of Related Art
An interferometric fiber optic gyroscope (IFOG) includes the main components of a light source, a beam splitter, a fiber optic sensing coil made of either polarization maintaining (PM) fiber or a low birefringence (standard telecommunications) fiber, a polarizer (sometimes more than one), and a detector for light from a light source which is split by loop beam splitter into counter-propagating waves traveling in the sensing coil. The associated electronics measure the phase relationship between the two interfering counter-propagating beams of light that emerge from opposite ends of the coil. The difference between the phase shifts experienced by the two beams is proportional to the rate rotation of the platform to which the instrument is fixed, due to the well known Sagnac effect.
Environmental factors can affect the measured phase shift difference between the counter-propagating beams, thereby introducing an error, such environmental factors include variables, such as temperature, vibration and magnetic fields. In general, such factors are unevenly distributed throughout the coil. These environmental factors induce variations in the optical light path that each counter-propagating wave encounters as it travels through the coil. The phase shifts induced upon the two waves are unequal, producing an undesirable phase shift which is indistinguishable from the rotation-induced signal.
Past approaches to reducing some of the sensitivities arising from environmental factors, have involved the use of a potting compound to pot the windings of the sensor coil within a matrix of an adhesive material. Such an approach is described in U.S. Pat. No. 5,321,593 for xe2x80x9cSensor Coil for Low Bias Fiber Optic Gyroscopexe2x80x9d, assigned to the assignee of the present application, and U.S. Pat. No. 5,546,482 for xe2x80x9cPotted Fiber Optic Gyro Sensor Coil for Stringent Vibration and Thermal Environmentsxe2x80x9d, also assigned to the assignee of the present invention, both disclosures being incorporated herein by reference. Careful selection of the potting material (particularly in terms of elasticity) results in reduction of vibration-induced bias, coil cracking, degradation of h-parameter and temperature-ramp bias sensitivity. In these arrangements, the coil is formed on a spool of carbon composite material whose coefficient of expansion approximates that of the overlying fiber windings. By closely matching the thermal expansion characteristics of the spool and the fiber windings as well as properly selecting the coil potting material, the Shupe-like bias caused by thermal stress that would be otherwise exerted by a standard metallic spool is minimized.
While the use of adequate potting and spool materials will tend to minimize bias environmental sensitivities, conventional support and spool designs, which feature a substantially-cylindrical hub sandwiched between a pair of end flanges, are difficult to xe2x80x9cmatchxe2x80x9d to the potted coil. This is due to the asymmetry of expansions of such coils in response to temperature change. The coefficients of thermal expansion of a potted coil in the axial direction is often on the order of one-hundred (100) times that of the radial direction. Unfortunately, a corresponding asymmetry does not exist with regard to the supporting spool. Rather, spools of conventional design and material composition exhibit isotropic thermal expansion characteristics. This relative imbalance introduces bias errors through coil stressing and creates bonding and cracking problems with the potting material in the potted coil. For example, in a spool-and-coil arrangement in which the material of the hub of the spool closely approximates the radial coefficient of the thermal expansion of the potted coil, the axial expansion of the coil will exceed that of the hub. As a result, significant axial compression of the coil can occur when the temperature rises since axial expansion of the potted coil is limited by a relatively xe2x80x9cfixedxe2x80x99 separation distance between the spool""s end flanges. Further, the stressing due to differential thermal expansion coefficients at the coil-hub interface can result in either rupture or in coil cracking. On the other hand, in a spool fabricated of material closely matching the axial coefficient of thermal expansion of the potted coil, one may expect the relatively-greater radial expansion of the hub in response to temperature change to degrade performance by squeezing the fiber of the coil whose radial dimension is relatively fixed.
In order to address the thermally-induced Shupe bias that results from the aforementioned thermal incompatibility of conventional spool designs with the asymmetric radial and axial thermal expansion coefficients of potted sensor coils, there have been attempts at designing spools with a single mounting flange and an interior hub. In these arrangements, the coil is mounted on the hub with its central axis traverse to the plane of the flange so that the coil is free to expand axially without the constraint of a second flange on the opposite side of the potted coil from the single mounting flange. For instance, U.S. Pat. No. 5,545,892 of Bilinski et al., entitled xe2x80x9cGyro Sensor Coil with Low-Friction Hub Interfacexe2x80x9d, also assigned to the assignee of the present invention, discloses a single mounting flange and central hub assembly having the central hub coated with a non-adhesive material. The non-adhesive coating provides a relatively friction-free surface upon which the innermost layer of the potted coil is free to slide to accommodate its significant axial expansion when subject to heating. The disclosure of this reference is hereby incorporated by reference.
Although this approach of using a non-adhesive coating on the central hub allows the potted coil to expand axially in response to temperature changes, the potted coil is not mechanically connected to the central hub and can become physically separated from the surface of the non-adhesive coating when the potted coil experiences thermal expansion or compression due to certain temperature changes. This mechanical separation causes the potted coil to become thermally disconnected from the central hub, which can result in sudden changes in the Shupe-driven bias behavior of the IFOG. Further, the separation of the potted coil from the central hub subjects it to vibration-induced bias effects that result from the free-standing arrangement of the potted coil relative to the spool. Such bias effects can become particularly acute in an environment that includes vibrations at the resonant frequency of the potted coil. Moreover, the operative mechanism of such device for overcoming the effects of axial coil expansion relies upon the ability of the potted coil to slide freely upon the surface of the hub. Even slight imperfections in the coating on the hub can on occasion cause a deleterious so-called xe2x80x9cstick and slipxe2x80x9d phenomenon. This effect can cause irregular and quasi-periodic stressing of the coil fiber resulting in unpredictable bias errors in the gyro output.
The foregoing shortcomings and disadvantages of the prior art are alleviated by the present invention that provides a rotation sensor for use in a fiber optic gyroscope including a centrally-located cylindrical hub having an improved hub-to-coil slip interface positioned there around. The central hub has a substantially planar mounting flange extending from one of its ends, where the axis of rotation of the central hub is orthogonal to the plane of the mounting flange. A sensor coil comprising a plurality of layers of coaxial turns of optical fiber embedded in a potting material is formed around an outer surface of the slip interface. The slip interface allows the potted coil to expand or contract along the axial direction of the central hub due to thermal expansion while maintaining a constant thermal and mechanical connection between the interface and the potted coil. In this manner, the present invention maintains the integrity of the connection of the potted coil to the slip interface during thermal expansion of the potted coil, thus minimizing temperature-induced Shupe bias errors in the rotation sensor.